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gray code encoding

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"Gray code encoding" refers to a type of encoding scheme based on a numbering system that assigns certain bit value(s) to a range of threshold voltages that a memory cell may have. The bit values are assigned such that the bit pattern differs between adjacent threshold voltage ranges by only one bit. Such a bit pattern assignment is advantageous because while a threshold voltage may change, drift, from one range to a neighboring range, unintended change or drift is likely not enough to cause a two bit change in the encoding. In this manner, undesired drift can be detected and accommodated.

In one example, the range of threshold voltages may be one of a plurality of threshold voltage ranges that may be used to encode multiple bits of data into a memory cell.  For example, suppose a memory cell is configured to store two bits of information, and the successive ranges of threshold voltages are between a negative lower bound threshold voltage and positive upper bound threshold, e.g., about 5 volts.  If four ranges are defined the lower most range may have a bit assignment of ‘00’, the next highest a bit assignment of ‘01’, the next highest a bit assignment of ‘10’, and the last range a bit assignment of ‘10’.

Of course, memory cells that store multiple bits of data may use a gray code encoding or another encoding process, such as those described in US patents: US6222762 and/or US7237074 which are included herein by reference, for all purposes.